Long QT syndrome (LQTS) is a disorder characterized by delayed cardiac repolarization and an increased risk of arrhythmias and sudden death. Long QT syndrome type 2 (LQT2) is caused by mutations in the human ether-a-go-go-related gene (hERG). hERG encodes the pore-forming subunit of the rapidly activating delayed rectifier potassium channel in the heart. LQT2 is the second most prevalent form of LQTS, accounting for 35% to 40% of genotyped cases of LQTS. LQT2 mutations can cause hERG channel dysfunction by a variety of mechanisms. In the previous funding period, we have shown that nonsense-mediated mRNA decay and splicing defects are important mechanisms of hERG channel dysfunction in LQT2. We have also shown that generation of hERG C-terminal isoforms is determined by competition between alternative splicing and polyadenylation of hERG intron 9 and that the relative expression of hERG C-terminal isoforms plays an important role in regulation of hERG channel function. In the present application, we will use full-length hERG gene constructs to study mechanisms that underlie the regulation of hERG C-terminal isoform expression, characterize two new mechanisms of hERG channel dysfunction in LQT2, and develop a novel approach to modulate the relative expression of hERG C-terminal isoforms. In addition, we will use patient-specific induced pluripotent stem (iPS) cell-derived cardiomyocytes as a model to study pathophysiology of LQT2. The specific aims of this application are: Aim 1) To study a newly identified LQT2 splice site mutation that disrupts the 3' splice site of intron 9 and alters the relative expression of hERG C-terminal isoforms. Aim 2) To develop an antisense approach to increase hERG current by inducing a shift in hERG C-terminal isoform expression from the nonfunctional isoform to the functional isoform. Aim 3) To characterize a new mechanism of LQT2 in which premature termination followed by the reinitiation of translation results in the generation of N-terminally truncated hERG channels with altered gating properties. Aim 4) To create LQT2 patient-specific iPS cell lines and characterize LQT2 mutations in iPS cell-derived cardiomyocytes. This study will increase our knowledge of how LQT2 mutations lead to hERG channel dysfunction at the posttranscriptional and translational level. We believe that this work will have a sustained and significant impact on our understanding and treatment of long QT syndrome.